Just began this last night-I've read it at least twice before but it remains a classic!
Basically a large body,presumed to be an asteroid, is discovered near Jupiter in 2131 and gven the designation 31/439 after the year of discovery and order of discovery.
However upon further probing by earth and space based telescopes it is discovered that the object is an unusal asteroid. For a start it is doesnt have a regular orbit, is about 40 kilometeres across and yet has a rotation period of about 4 days! The object is spinning very fast!
Soon it is given the name Rama after a Hindu god and an expedition is sent out to meet it and find out just what it is....
I first read this many years ago and although it was written in 1973 it remains compelling with good science and not bad characters.
There were 3 sequels to the book but really you don't need to bother (indeed some will say you shouldnt bother, as they're mostly written by Gentry Lee) but this fine book stands the test of time and deserves to be appreciated in its own right!